Why b not change its value?

Question

a=[1,[2,3]]
b=a[:]
a[0]=3
print(b)

Answer 1

b will not charge because both a and b are references to different values.If you get confused try to print the id of a and b
print(id(a))
print(id(b))

Answer 2

Why it will change
You assigned b =a[:]
Which means b contain the value from beginning to end
and if you want to change value then change value before passing value to b